The present invention relates to the field of switching power transfer devices, and more particularly to an improved energy recovery method for a flyback converter.
The leakage inductance of the transformer in a conventional DC/DC flyback converter causes a voltage spike across the power switch when the power switch turns off Usually a circuit, such as a R-C-D (resistor, capacitor, diode) snubber circuit or an active clamp circuit, is used to absorb this voltage spike. The leakage energy of the transformer is typically dissipated in the R-C-D snubber circuit.
A number of known designs seek to recover this energy. These methods typically require an additional active switch to recover the leakage energy of the transformer.
A well-known conventional DC/DC flyback converter is shown in FIG. 1, where Lk is the leakage inductance of the transformer T. The typical switching waveforms of FIG. 1 are shown in FIG. 2. When switch S is turned off at t2, the leakage current charges the parasitic output capacitance of switch S (output capacitance of S is not shown in FIG. 1), which causes a high voltage spike across switch S. After the leakage energy is completely released, the voltage across switch S reaches its steady-state value. As a result, a high voltage rating voltage switch S would be required.
To eliminate this voltage spike, a number of circuit topologies have been reported in the literature. Among them, the R-C-D snubber is one of the most popular ways to minimize the voltage spike as shown in FIG. 3. The snubber circuit consists of diode D1, capacitor Cs and resistor Rs. When switch S is turned off, the leakage current flows through diode D1 and charges capacitance Cs. If capacitance Cs is relatively large enough, the voltage across Cs roughly does not change, so as to clamp the voltage. In this case, the leakage energy of the transformer is first charged to Cs and is then dissipated by the resistor Rs. As a result, the voltage clamp is achieved at the expense of low conversion efficiency, i.e., loss of the energy inherent in the spike to heat.
Another prior art circuit is shown in FIG. 4. See, Moshe Domb, xe2x80x9cNondissipative turn-off snubber alleviates switching power dissipation second-breakdown stress and Vce overshoot: analysis, design procedure and experimental verification,xe2x80x9d IEEE Power Electronics Specialists Conference (1982). In this circuit, when switch S is turned off, the leakage energy of the transformer T is transferred to the clamping capacitor Cs through D1. The voltage stress across switch S is the sum of the input voltage Vin and the clamping voltage Vc across Cs, which is expressed as follows
Vds,max=Vin+Vc.
When switch S is turned on, the clamping capacitor Cs and inductor Lr form a resonant tank. The energy stored in capacitor Cs is transferred to the inductor Lr and the voltage polarity across capacitor Cs reverses due to the resonance. When the capacitor voltage vc reverses and reaches the input voltage Vin, diode D1 conducts. The energy stored in Lr is delivered to the input source. Therefore, the leakage energy of the transformer is finally feedback to the input source. In this scheme, an additional separate inductor Lr is required, which increases the cost. See, U.S. Pat. Nos. 4,783,727, xe2x80x9cDC/DC Converterxe2x80x9d; 6,115,271, xe2x80x9cSwitching Power Converters With Improved Lossless Snubber Networksxe2x80x9d, 5,260,607, xe2x80x9cSnubber Circuit For Power Converterxe2x80x9d, each of which is incorporated herein by reference.
Another well-known prior art method provides an active clamp, as shown in FIG. 5. See, R. Watson, F. C. Lee and G. C. Hua, xe2x80x9cUtilization of an active clamp circuit to achieve soft-switching in flyback convertersxe2x80x9d IEEE Power Electronics Specialists Conference (1994). An active switch Sa and a capacitor Cs are provided in series and connected in the primary winding N1 of the transformer T. When switch S is turned off, switch Sa is turned on. The leakage energy is transferred to the capacitor Cs through switch Sa, and the voltage across Cs is used to reset the transformer. As a result, the voltage across switch S is clamped. However, such a converter requires an additional active switch and its controller. It increases the cost, which is not desirable for manufacturers.
See, U.S. Pat. No. 4,675,796, expressly incorporated herein by reference, discussed below. See also, Farrington, U.S. Pat. No. 5,883,795 and Farrington, U.S. Pat. No. 5,883,793, expressly incorporated herein by reference.
U.S. Pat. No. 6,108,218, xe2x80x9cSwitching Power Supply with Power Factor Controlxe2x80x9d, provides two embodiments. In a first embodiment, shown in FIGS. 1 and 2 thereof, no snubber circuit to recycle the leakage energy of the transformer is shown. FIGS. 3 and 4 provide an additional active switch as part of the snubber.
U.S. Pat. No. 5,982,638, xe2x80x9cSingle stage power converter with regenerative snubber and power factor correctionxe2x80x9d provides a capacitor 44 in FIG. 1, which is not only used as a snubber capacitor, but also used to achieve power factor correction, and therefore handles the main power flow from the input to the output. Therefore, the current flowing through this capacitor 44 is very large, which requires a capacitor large in size and value. In this circuit, the recovery of energy from the snubber capacitor 44 occurs by transfer to the input inductor 38 when switch 22 turns on. Since the energy stored in capacitor 44 is large, which causes higher power loss in the circuit. As a result, it has lower power conversion efficiency. The capacitance of capacitor 44 is determined by the input power and satisfies the power factor and input current harmonics requirements.
U.S. Pat. No. 5,991,172, xe2x80x9cAC/DC flyback converter with improved power factor and reduced switching loss,xe2x80x9d provides a third transformer winding which is not used to recover the leakage energy of the transformer, but rather to reduce the switching loss and improve the power factor. The leakage energy is dissipated by the circuit. Thus, it provides no substantial improvement in efficiency over a dissipative R-C-D snubber.
U.S. Pat. No. 5,999,419, xe2x80x9cNon-isolated Boost Converter With Current Steeringxe2x80x9d relates to a buck boost converter having a tree-winding transformer.
U.S. Pat. No. 5,896,284, xe2x80x9cSwitching Power Supply Apparatus With a Return Circuit That Provides A Return Energy Ro A Loadxe2x80x9d, relates to a power supply circuit which utilizes leakage inductance energy to enhance efficiency, for example with a magnetically isolated inductor.
U.S. Pat. No. 5,615,094, xe2x80x9cNon-Dissipative Snubber Circuit For A Switched Mode Power Supplyxe2x80x9d, relates to a snubber circuit for a secondary circuit of a power supply.
U.S. Pat. No. 5,694,304, xe2x80x9cHigh Efficiency Resonant Switching Convertersxe2x80x9d; and U.S. Pat. No. 5,379,206, xe2x80x9cLow Loss Snubber Circuit With Active recovery Switchxe2x80x9d each provide a dual active switch architecture converter.
U.S. Pat. No. 5,055,991, xe2x80x9cLossless Snubberxe2x80x9d, relates to a converter circuit having an active switch and a transformer with five inductively coupled windings.
U.S. Pat. No. 5,019,957, xe2x80x9cForward Converter Type of Switched Power Supplyxe2x80x9d, relates to a dual active switch forward power converter.
U.S. Pat. No. 4,805,079, xe2x80x9cSwitched Voltage Converterxe2x80x9d, provides a converter with a snubber circuit.
U.S. Pat. No. 4,760,512, xe2x80x9cCircuit for Reducing Transistor Stress and Resetting the Transformer Core of a Power Converterxe2x80x9d, relates to a single active switch, triple inductively coupled winding transformer forward converter.
U.S. Pat. No. 4,736,285 relates to a xe2x80x9cDemagnetization circuit for Forward Converterxe2x80x9d, having two active switches.
U.S. Pat. No. 4,688,160, xe2x80x9cSingle Ended Forward Converter With Resonant Commutation of Magnetizing Currentxe2x80x9d, provides a forward converter employing a resonating capacitor to reset the transformer core.
U.S. Pat. No. 4,561,046, xe2x80x9cSingle Transistor Forward Converter With Lossless magnetic Core Reset and Snubber Networkxe2x80x9d, relates to a forward converter having a single switch and a transformer having three inductively linked windings.
U.S. Pat. No. 4,441,146, xe2x80x9coptimal Resetting of the Transformer""s Core in Single Ended Forward Convertersxe2x80x9d, provides a forward DC/DC converter having a transformer with three inductively coupled windings.
U.S. Pat. No. 4,355,352, xe2x80x9cDC To DC Converterxe2x80x9d, relates to a converter having three coupled inductor windings, with two capacitors and two switching devices (one active and one passive), to provides a ripple free input and output current.
The present invention provides a switching power circuit, in which leakage energy of a flyback transformer is efficiently recycled to the output and voltage spike across the switch reduced, by clamping the voltage across the switch using an winding-capacitor-diode snubber inductively coupled to the flyback transformer.
Accordingly, the voltage across the main switch due to the leakage inductance of the transformer is clamped, achieving a reduction in peak voltage across the switch, and the energy inherent in the voltage spike is recaptured, to increase overall circuit efficiency. Further, the circuit may be constructed using a single active switch and a single inductively coupled transformer structure. Thus, the cost is low and the circuit takes up little additional space.
The snubber capacitor Cs only deals with the leakage energy, which is only about 2% of total power handled by the circuit. This allows use of a relatively small snubber capacitor. In addition, the size of the capacitor Cs is independent of the input power level.
The leakage energy is recovered through an extra winding of the flyback transformer, and becomes the part of the magnetizing energy when the main switch S is turned on.
Thus, it is an object of the invention to provide an winding-capacitor-diode snubber for a flyback converter providing voltage clamping across the active switch and which recycles energy from the clamped transient, without requiring a separate active switch nor separate inductor.
It is also an object of the invention to provide a flyback converter circuit having an active switch, diode, capacitor, flyback transformer and an winding-capacitor-diode snubber for reducing a voltage spike transient across the active switch, wherein the winding-capacitor-diode snubber is configured to transfer energy from the voltage spike transient to the output through an inductor coupled to the flyback transformer. The snubbing energy recovery switching is passive, through the diodes, and therefore no additional active switch is required.
It is a further object of the invention to provide a method of operating a flyback converter for improved efficiency, the flyback converter circuit having an active switch, diode, capacitor, flyback transformer and an winding-capacitor-diode snubber for reducing a voltage spike transient across the active switch, wherein the winding-capacitor-diode snubber passively switches to transfer energy from the voltage spike transient to the output through an inductor coupled to the flyback transformer.
According to a boost-flyback switching converter embodiment of the invention an energy efficient snubber circuit is provided, having an input inductor and an actively controlled switch, comprising a transformer having a primary, secondary and auxiliary winding, a clamping capacitor and a first passive switch in series across the primary winding, the auxiliary winding of the transformer and a second passive switch being connected, in series, to the node between the clamping capacitor and first passive switch and ground, and the active switch is connected between ground and a side of the primary winding opposite the first passive switch. The circuit may also be configured as a power factor correction circuit.
According to a method of the present invention for efficiently snubbing an active switch in a converter circuit, having a transformer, in which a magnetizing energy and leakage inductance energy are supplied to a transformer, and a clamping capacitor discharged during a conducting state of the active switch, and the magnetizing energy and leakage inductance energy transferred to the load, and the clamping capacitor charged during a nonconducting state of the active switch. Thus, the circuit including the secondary winding of the transformer is configured to receive energy when the active switch is turned off. The clamping capacitor clamps the voltage across the active switch during a turn-off transient. The leakage inductance energy, passing through the clamping capacitor, is recycled to the load through an auxiliary winding of the transformer, to reset the transformer. The circuit thus efficiently transfers energy, including energy corresponding to a switching transient of the active switch, to the load, while limiting maximum active switch voltage and properly resets the transformer each cycle, employing only a single active switch and a single inductive component. The circuit may, of course, include further components.
A control circuit for controlling the active switch is of known type, and for example, provides operation in a pulse-width modulation mode. The output circuit, for example, includes the secondary winding connected to a single diode with a capacitor filter, although more complex designs may be employed.
The passive switches are, for example, semiconductor diodes, while the active switch is, for example, a metal-oxide-silicon field effect transistor. The transformer is a flyback transformer, having an auxiliary winding. The various transformer windings are preferably electrically isolated, and the design is compatible with circuit designs having a plurality of secondary windings.
A capacitor is preferably provided in series between said active switch and said primary winding, for example having a voltage waveform that is out of phase with the clamping capacitor waveform. The auxiliary winding of the transformer and the second passive switch are preferably in series with the first passive switch across the capacitor; the active switch preferably is in series with a magnetically isolated inductor, across an input voltage source; and the auxiliary winding of the transformer and the second passive switch are preferably connected through the first passive switch to a common node of the primary winding and a ground-referenced capacitor. Various modifications of the arrangement are possible, which do not essentially alter the function of the circuit to limit voltage across the switch and efficiently recapture energy from the turning off of the active switch. During an active switch OFF steady state condition, i.e., after settling of transients, the first and second passive switches are reverse biased. During the ON state, leakage energy from the primary winding is transferred from the clamping capacitor to the auxiliary winding as a part of the magnetizing energy, whereby said leakage energy is inductively coupled to the secondary winding. The active switch may include an intrinsic diode or one may be provided externally or separately. Thus, the auxiliary transformer winding transfers energy from a switching transient of the active switch to the secondary winding.
In a power factor correction embodiment, the active switch is connected in series with an inductively isolated inductor to a rectified output of a full wave rectifier.
These and other objects will become apparent from a review of the Detailed Description of the Preferred Embodiments and drawings.